Multi-shaft turboprop engines for aeronautical use are provided with a compressor that draws in air coming from an air intake and drives, through a first shaft, a first stage of a turbine that is set in rotation by exhaust gases coming from a plurality of burners. The turbine is provided with a second stage that drives the propeller by means of a second shaft, which is usually mounted coaxially inside the first shaft.
The starting of these multi-shaft turboprop engines is performed by means of an electric starter, in which an electric motor is connected to the first shaft to set the compressor and the first stage of the turbine in rotation. U.S. Pat. No. 4,799,354 illustrates an example of the above-mentioned type of starting system.
The energy necessary for this starting operation is taken from a plurality of batteries present on-board the aircraft or installed in an external unit positioned, on the runway near the aircraft.
Starting operations of the turboprop engine are easy on the runway, but can become extremely critical in flight, as the energy available on board can be limited in many operating conditions.
In addition, in such conditions, the rotational energy deriving from propeller being driven by the forward movement of the aircraft (known as wind-milling) cannot be transferred to the compressor and used to make it turn because the above indicated structure does not provide for any direct connection between propeller and compressor.
Rotation of the compressor with the turboprop engine switched off and by dynamic effects is therefore only possible by using air coming from the air intake; this action can be used to increase the probability of restarting the turboprop engine by means of The electric starter. Unfortunately, the dynamic action of air drawn in through the air intake is only effective for a limited range of aircraft altitudes/speeds.
The need is therefore felt to provide an aeronautical turboprop engine in which in-flight starting operations are improved with respect to those available in the known art.